I had a minor stroke in march been having bad headaches had another mri and now they say it was a major stoke how can a stroke grow?

Major vs minor: There is no definition of major vs minor stroke. There is the possibility that you've had another stroke or it may simply be that there is confusion in the terminology and that the stroke has not gotten worse but rather just undergone the expected healing process. The most important question is, are your symptoms worse? If not, the appearance of the stroke is likely not critical.

Stroke extension: The vessel responsible for your stroke may have been partially occluded initally, giving you a minor stroke. In the interval period of time, because of reduced flow and/or other factors, the vessel may have totally clotted, extending the area of infarcted tissue seen on the second mri, and now classifying the stroke as a major event. Progression of symptoms during the interval period is key!